Adhesion of leukocytes to vascular endothelium is one of the earliest events in a variety of immune-inflammatory reactions. The process participates in vascular occlusions and contributes to atherothrombotic lesions. At the molecular level, leukocyte adhesion to endothelial cells is a redundant mechanism, supported by the regulated recognition of a disparate set of membrane receptors, including integrins, expressed on both leukocytes and resting or cytokine-activated endothelial cells.
Integrins are a functionally and structurally related group of receptors that interact with a wide variety of ligands including extracellular matrix glycoproteins, complement and other cells. Integrins participate in cell-matrix and cell-cell adhesion in many physiologically important processes including embryological development, hemostasis, thrombosis, wound healing immune and nonimmune defense mechanisms and oncogenic transformation. See Hynes, Cell, 48:549-554 (1987). The majority of integrins participating in dynamic cell adhesion, bind a tripeptide, arginine-glycine-aspartic acid (RGD), present in their ligand, causing cell adhesion. See Ruoslahti et al., Science, 238:491-497 (1987).
Mac-1 (CD11b/CD18) is an integrin receptor found predominantly on macrophages and granulocytes. Like all integrin receptors, Mac-1 is a heterodimeric, transmembrane glycoprotein composed of non-covalently associated alpha and beta subunits.
Mac-1 mediates neutrophil/monocyte adhesion to vascular endothelium and phagocytosis of complement-opsonized particles. Antibodies to the Mac-1 receptor alter neutrophil function in vivo including inhibiting neutrophil migration into inflammatory sites. See Price et al., J. Immunol., 139:4174-4177 (1987). Mac-1 also functions as a receptor for fibrinogen in a reaction linked to fibrin deposition on the monocyte surface. See Altieri et al., J. Cell Biol., 107:1893-1900 (1988); Wright et al., Proc. Natl. Acad. Sci. USA, 85:7734-7738 (1988); Trezzini et al., Biochem. Biophys. Res. Commun., 156:477-484 (1988) and Gustafson et al., J. Cell Biol., 109:377-387 (1989).
Fibrinogen is a complex molecule of approximately 340,000 daltons and consists of three pairs of subunit polypeptides, called the .alpha., .beta. and .gamma. chains. These individual chains are held together by several disulfide bonds. The proteolytic digestion of fibrinogen by plasmin produces fragments A, B, C, D and E, all having a molecular weight of less the 5,000 daltons. See Pizzo et al., J. Biol. Chem., 47:636-645 (1972).
Further proteolytic digestion of fibrinogen by plasmin produces a D.sub.30 fragment with a molecular weight of about 30,000 daltons containing portions of the .alpha., .beta. and .gamma. chains of fibrinogen. See Furlan et al., Biochim. Biophys. Acta., 400:95-111 (1975).
The deposition of fibrinogen on the leukocyte surface occurs in a variety of inflammatory responses such as delayed type hypersensitivity, incompatible transplant rejection and the physiopathology of vascular obstruction and atherogenesis. See Geczy et al., J. Immunol., 130:2743-2749 (1983); Hooper et al., J. Immunol., 126:1052-1058 (1981); Colvin et al., J. Immunol., 114:377-387 (1975); Hattler et al., Cell Immunology, 9:289-295 (1973); Gerrity, R. G., Am. J. Pathol., 103:181-190 (1981) and Am. J. Pathol., 103:191-200 (1981); and Shelley et al., Nature, 270:343-344 (1977).
Interactions of fibrinogen on cell surface receptors of endothelial cells have been described. Languino et al., Blood, 73:734 (1989) describe the binding of fibrinogen to endothelial cells by an RGD-dependent mechanism. It is generally believed that the vitronectin receptor is the major endothelial receptor for fibrinogen. Cheresh et al, Proc. Natl. Acad. Sci. USA, 84:6471-6475 (1987). Other endothelial cell receptors reported to bind fibrinogen include cell surface bound transglutaminase, and an 130 kilodalton receptor that binds to fibrin peptides. Erban et al., J. Biol. Chem., 267:2451 (1992).
Also on the surface of endothelial cells is an intercellular adhesion molecule 1 (ICAM-1) that has been described by Springer, Nature, 346:425-433 (1990), and has been shown to bind the leukocyte integrin LFA-1.
Recently, the interaction of fibrinogen with the Mac-1 receptor of leukocytes has been shown to be a dynamic cell adhesion reaction involving the recognition of the tripeptide RGD within fibrinogen by the Mac-1 receptor similar to the interaction of fibrinogen with the integrin receptors on platelets and endothelial cells. See Altieri et al., J. Clinic Invest., 78:968-976 (1986); Pytela et al., Science, 231:1559-1562 (1986); Ruoslahti et al., Science, 238:491-497 (1987); Ruoslahti et al., Cell, 44:517-518 (1986); and International PCT Application No. PCT/US91/05096.